As semiconductor feature sizes continue to shrink into the sub-micron range, the effects of light diffraction during photolithographic processes become more pronounced. Common ultraviolet (UV) exposure tools use light sources having a 193, 248, or 365 nm wavelength. When such tools are used to form semiconductor devices having feature sizes of 70 to 350 nm, the effects of light diffraction become quite pronounced and produce interference that affect printed patterns on the semiconductor device.
One area where interference effects are particularly problematic is with patterns that are placed orthogonally to each other. The interference effects tend to degrade the aerial image, thus limiting the resolution and the photolithography process window. The effects of interference can be seen especially at the edges of array type patterns where lines tend to merge or fall over in these regions more easily. In particular, the interference effects appear as side maxima in the printing of the patterns on a semiconductor wafer, which adversely affect the resolution of printed pattern.
There are prior art solutions that use various methods to improve overall resolution of patterns. Some of the prior art solutions utilize off-axis illumination or phase shifting masks. Off-axis illumination will increase overall resolution by re-focusing some of the light rays that are generally diffracted outside of the line of projection. Phase shifting masks shift the phase of a light beam to produce incoherent light where there is a phase difference between different features. The resolution of features is thus improved. However, the present invention improves the lithographic imaging quality of orthogonally oriented features beyond that which is accomplished by applying only the prior art solutions mentioned above. The present invention consists of a less costly solution that is simple and requires no change to existing photolithographic optics or equipment.